1. Field of the Invention
This invention relates to gas-liquid contacting devices and the use of such devices in liquid treatment systems. The invention additionally relates to oxygen-absorption processes requiring repeated and prolonged air-liquid contact in sequential stages. The invention especially relates to methods and apparatus for aeration pumping of waste water such as sewage within aerobic purification systems of the looped channel type, such as oxidation ditches.
2. Review of the Prior Art
Many liquid treatment processes, commonly termed aerobic processes, supply bacteria and other bio-organisms with dissolved oxygen for treating aqueous wastes such as municipal sewage, cannery wastes, dairy wastes, meat-processing wastes, and the like.
Such aerobic processes are commonly accelerated by concentrating and activating the bio-organisms, termed bio-mass or activated sludge, and returning this sludge to be mixed with incoming wastewater which supplies food for the organisms. Activated-sludge processes for aerobic treatment of wastewaters have followed two main lines of development: vertical-flow aeration basins and circuit-flow oxidation ditches.
Vertical-flow aeration basins are typically aerated on a large scale with one or more impeller-type aerators which are vertically mounted and disposed at the surface of the liquid, as discussed in Water & Wastes Engineering, September 1975, pages 76-79, using an aerator such as is described in U.S. Pat. No. 3,479,017 and producing a uniform dissolved-oxygen (D.O.) content of 2.0 mg/liter. Such impeller aerators are frequently mounted within and at the upper open end of a draft tube extending partially or entirely to the bottom of the basin so that the aerator can more efficiently pump liquid from the bottom of the basin, having a depth up to 40 feet, and disperse it over the surface of the basin, thereby improving vertical circulation over a wide area. When fitted with a gear reducer to spin a nine-foot diameter impeller at low speeds, oxygen transfer efficiencies of 3.5 pounds O.sub.2 /hp/hour have been approached.
In an early oxidation-ditch process, Dutch Pat. No. 87,500 discloses horizontally mounted rotors having brush surfaces for adding oxygen to sewage and causing the sewage to flow for a period of time in a closed-loop circuit within an ovally laid-out ditch, the liquid then being clarified by settling and excess sludge being removed. In subsequent developments directed to adding oxygen to sewage and inducing circuit-flow circulation in oxidation ditches, U.S. Pat. No. 3,336,016 discloses an S-shaped duct, U.S. Pat. No. 3,510,110 the combination of a longitudinal partition and a vertically disposed surface aerator which is adjacent thereto, and U.S. Pat. No. 3,846,292 a plurality of subsurface ejector aerators.
Finally, U.S. Pat. No. 3,900,394 discloses a sewage purification process, to be carried out in a circuit-flow oxidation ditch having an impeller-type aerator at one or both ends, which comprises sequential aeration of incoming sewage, aerobic decomposition and depletion of its oxygen content, introduction of additional sewage to the oxygen-starved bacteria, and, simultaneously, aerobic decomposition and denitrification of the additional sewage as the bacteria break down its nitrates.
A circuit-flow oxidation ditch is a complete mix system operating in plug-type flow. It can be designed to operate with recycled sludge on a food-to-microorganism ratio (F/M) varying over a possible range of 0.01 to 5.0, depending upon space, cost, and process design requirements. If operating at a low F/M ratio of 0.01-0.2, it is an extended aeration system, producing small quantities of sludge. If operating at a medium F/M ratio of 0.2-0.5, it is a conventional system. If operating at a high F/M ratio of 0.5-2.5, it is a high-rate activated sludge system, producing large quantities of sludge. Moreover, it can even be operated as an activated lagoon with no recycled sludge, having F/M ratios above 2.5. An oxidation ditch may also shift through a wide F/M range, representing all three of these systems, as it begins operation as a high-rate activated sludge system, with no built-up sludge, and gradually builds up its recycled sludge to a mixed liquor suspended solids (MLSS) content of 3,000 mg/l where extended aeration can generally be considered to begin.
There are now three main types of aeration apparatuses in use within circuit-flow oxidation ditches of varied depth and variety of layouts, such as an oval-shaped racetrack and a plurality of looped or endless channels. These are:
(1) horizontally shafted surface aerators,
(2) vertically shafted surface aerators, and
(3) eddy-jet type subsurface aerators.
Horizontally shafted surface aerators are of two general types:
(1) brush, cage, or rotor aerators, such as those manufactured by Lakeside Equipment Corporation, Bartlett, Ill., and Passavant Corporation, Birmingham, Ala., which have a maximum power input of about 50-60 horsepower, and
(2) aeration discs, such as those manufactured by Envirex, Incorporated, Waukesha, Wis.
Each type transfers 2-3.25 pounds of oxygen per shaft horsepower per hour from air to the mixed liquor.
Slow-speed surface aerators are used in looped channel designs known as Carrousel, developed by Dwars, Heederik Verhey, of Amersfoort, the Netherlands, as an improved form of the basic oxidation ditch. Each aerator is mounted vertically in the rounded end of a deep channel having a central partition that is positioned close to the aerator to create within the channel a uniform turbulent flow that is both longitudinal and spiral in nature. Such a surface aerator transfers 3-4 pounds of O.sub.2 /hp/hr from air to mixed liquor.
An eddy-jet system is known as deep channel jet aeration, sold by Penberthy Division of Houdaille Industries, in which air jet headers are mounted near the floor of a 20-foot deep channel to provide propulsion and high-efficiency aeration at an oxygen transfer efficiency of 3-4 pounds O.sub.2 /hp/hr.
Greater transfer efficiencies are needed in order to conserve energy and minimize the cost and number of aeration devices which are needed in an oxidation ditch.
When oxygenating water with air, the necessary driving force increases non-linearly as the dissolved-oxygen content of the water increases. Frequency of liquid-gas contact is consequently quite important from an efficiency viewpoint even though mixing of liquid parcels having various contents of dissolved oxygen soon produces a uniform average oxygen content. More specifically, if a portion of the liquid, initially having zero dissolved oxygen, contacts a gas such as air several times, it at first absorbs oxygen very readily but increasingly slowly thereafter. Vertical circulation causes some aerated water to be directly back-mixed into the intake of the aerator. Thus, energy is wasted by attempting to re-aerate water that has already been aerated. A need consequently exists for a flow control method and means for minimizing vertical circulation and turbulent mixing and for bringing liquid and gas into singly occurring contact.
These prior-art systems using surface aerators are generally plagued with aerosol spray and misting, freezing problems in cold weather, dependence of mixing and power consumption upon oxygen demand, excessive noise, dependence of power consumption upon liquid-level variations, and the need for floating aerators to compensate for variations in the liquid level. The eddy-jet system requires excessively high blower pressure to introduce air at the bottom of a deep channel and requires the operation and maintenance of a plurality of circulation pumps to force or inject mixed liquor through the submerged jets.
These problems could be minimized or obviated by using a submerged turbine to provide subsurface aeration, but there is no means available for mounting a submerged turbine within an oxidation ditch so that plug-type flow can be generated, channel velocity can be accurately controlled, back mixing can be avoided, and complete mix can be attained.
In order to facilitate mass transfer of oxygen from air bubbles into the mixed liquor and thence into the microorganisms, it is also desirable to avoid the prior-art environment of relative quiesence and to provide instead a means for shearing all of the bacterial floc within an oxidation ditch into smaller particles, such as by forcing all of the mixed liquor past a shear-type pump means or a bubble-splitting and mixing means, at least once per circuit-flow cycle. However, no means exists in the prior art for requiring all of the mixed liquor to pass through either such means.
In addition, bacterial activity can be enhanced by increasing the dissolved-oxygen content of the mixed liquor at least once per circuit-flow cycle, such as by generating pressures upon the air bubble-mixed liquor mixture that are greater than the hydraulic pressure within the channel of the oxidation ditch. Again, no such practical means exists for an oxidation ditch.